Petrology of the Hornfels Contact Zone around the Hrossatungur Gabbro in the Eroded Hafnarfjall Central Volcano, W Iceland

Abstract

This study focusses on a hornfels zone on the southern side of the Hrossatungur gabbro, within the Hafnarfjall-Skarðsheiði central volcano, W-Iceland. The intrusion formed by repetitive injections of basaltic cone sheets, which were trapped at a lava-pyroclastite boundary within the Hafnarfjall caldera fillings. During contact metamorphism in response to the intrusion emplacement, a hornfels contact zone was created by a recrystallization process of previously hydrothermally altered basalt. Here, the hydrothermal alteration and chemical composition of minerals is closely studied to evaluate the development during the contact metamorphism event forming the hornfels around the gabbro. The hornfels mainly contains clinopyroxene compositions ranging widely from augite, salite, ferrosalite to hedenbergite in vesicle fillings, with minor orthopyroxene, while the groundmass clinopyroxene ranges from diopside, augite to salite. The plagioclase composition ranges from andesine, labradorite to bytownite and occasionally to anorthite within the vesicles and veins, while the groundmass plagioclase ranges from labradorite to anorthite. Other minerals found in the hornfels are iron-titanium oxides (magnetite, ilmenite and titano-magnetite), garnet, titanite, minor apatite and hornblende. The orthopyroxene include bronzite (Fs10-30), hypersthene (Fs30-50) and ferro-hypersthene (Fs50-70). Garnet compositional range goes mainly from andradite to about 20% grossular. Loss-on-ignition measurements reveal that the majority of samples located at the inner hornfels zone experience <1% LOI, while samples that show more extensive LOI are mostly situated at the outer hornfels zone.

A comparison of the hornfels LOI with the LOI of Icelandic rocks in different alteration zones indicates that the hornfels rocks should have shown LOI >1% to <10% prior to the gabbro emplacement, which indicates that the water has been driven out of the rock by the replacement of hydrous minerals by non-hydrous minerals. For comparison, the mineralogy of the hornfels zone surrounding a dyke intrusion in drillhole HE-42 in Hellisheiði geothermal field was studied. There the clino-pyroxene ranges from augite to salite in veins and vesicles, with minor orthopyroxene in the groundmass, the plagioclase composition ranges from labradorite to bytownite and anorthite. The hornfels in the well was more intense than that found around the gabbro observed by more pronounced granoblastic crystallization, less iron rich clinopyroxenes and more calcium rich plagioclases. The study shows that the formation of the hornfels is due to direct heat conduction, the expulsion of water from the rock and preventing a direct water-magma interaction.

Þetta MS verkefni fjallar um rannsókn á hitaummyndað berg (hornfels) við jaðar gabbróhleifs (Hrossatungugabbró) innan megineldstöðvarinnar í Hafnarfjalli-Skarðsheiði í Borgarfirði. Gabbróið myndaðist í innskotahrinu keiluganga inn í kvikugildru á mótum hraunlagastafla og brotabergs innan öskjumyndunar. Grannberg gabbrósins einkennist af belti af hitaummynduðu bergi (hornfels) sem felst í endurkristöllun á ummynduðu basalti. Þessi endurkristöllun, sem var skoðuð í bergfræði- og rafeindasmásjám og örgreini sýnir meginsteindirnar klínó-pýroxen (salít-ágít-ferrosalít-hedenbergít), vott af ortho-pýroxen, plagíóklas (labradorít-býtownít-anorthít) og oxíð (magnetít-títanómagnetít-ilmenít), og síðan minna af granati, títaníti, amfíbóli, apatíti og súlfíðum. Þessi kristöllun rekur mest af vatninu og koldíoxíðinu úr berginu, sem sést vel af lágum gildum í glæðitapi. Efnagreiningar (ICP-OES) í bergsýnum sýna tiltölulega litlar breytingar miðað við ferskt berg úr eldstöðinni, nema etv. aukningu í Al2O3, Zn, Ni og Cu og síðan minnkun í Sr og Ba. Þessi takmörkuðu efnabreytingar tengjast líklegast svipaðrar endurkristöllunar og finnast í fersku basalti. Samanburðarrannsókn, sem gerð var á hitaummynduðu bergi við gangajaðar á um 1500 m dýpi í holu HE-42 á Hellisheiði, sýnir mjög svipaðar niðurstöður og fengust við jaðar gabbrósins. Megin rannsóknaspurning þessa verkefnis var um tengsl vatns og kviku við jaðra gabbrósins. Niðurstöður benda eindregið til að myndun hitaummyndaða bergið (hornfels) sé sterk vísbending um að hiti færist frá gabbróinu til grannbergsins með hitaleiðni og loki fyrir aðgang vatns.